1. Field of the Invention
The present invention relates to a motion measuring apparatus for a ball such as a golf ball or a tennis ball. More particularly, the present invention relates to an apparatus for measuring the motion of a ball by utilizing image data obtained by a CCD camera.
2. Description of the Related Art
When a golf ball is hit with a golf club, it flies with a so-called backspin. The backspin is a rotation setting a horizontal direction orthogonal to a hitting direction (which will be hereinafter referred to as a “z direction”) to be an axis. A lift acts on the golf ball through the backspin, thereby increasing a flight distance of the golf ball. In some cases, the golf ball flies with a so-called sidespin. The sidespin is a rotation setting a vertical direction (which will be hereinafter referred to as a “y direction”) to be an axis. The golf ball turns left (a draw ball for a right-handed golf player) or turns right (a fade ball for the right-handed golf player) due to the sidespin. Furthermore, the golf ball sometimes flies with a rotation setting a horizontal direction identical to the hitting direction (which will be hereinafter referred to as an “x direction”) to be an axis.
A direction of the spin and a rotating speed greatly influence the subsequent trajectory of the golf ball. Similarly, the flight direction and flight speed of the golf ball greatly influence the subsequent trajectory. The measurement of the spin, the flight direction and the flight speed is effective for diagnosing the swing form of a golf player. Moreover, the measurement is also effective for the evaluation of a golf ball and a golf club. In a stage in which the golf ball and the golf club are developed, the measurement is inevitable.
Japanese Unexamined Patent Publication No. Hei 3-210282 (1991/210282) has disclosed an apparatus for photographing a flying sphere twice by means of a camera and measuring the flight speed of the sphere based on two images thus obtained. In the apparatus, the moving distance of the sphere is obtained from the two images and is divided by a photographing time interval so that the flight speed is calculated.
Japanese Patent No. 2810320 has disclosed a measuring method for photographing a flying golf ball twice at a predetermined time interval and calculating a spin rate from two images thus obtained. In the measuring method, a recognition mark put on the surface of the golf ball is read from first and second images and a rotating angle from the first image to the second image is obtained based thereon. The spin rate is calculated by the rotating angle and the photographing time interval.
Japanese Unexamined Patent Publication No. Hei 10-186474 (1998/186474) has disclosed a measuring method for photographing a flying sphere by means of two cameras in a shifted timing and calculating a spin rate, a flight speed and a flight direction from two images thus obtained.
In the case in which the flying golf ball is to be photographed by means of a camera, it is necessary to predict the position of the golf ball when a predetermined time passes after hitting and to determine the position of the camera such that the same position is included within a visual field. The head speed of a golf club is greatly varied depending on a golf player. Moreover, there are various golf clubs from a driver (W1) to a wedge. A head speed and a launch angle are greatly varied depending on the type of the golf club. Accordingly, the position of the golf ball after a predetermined time passes since hitting is considerably changed for each golf player or each used club. Therefore, it is not easy to predict the position of the golf ball after the predetermined time passes since the hitting. Even if the prediction can be carried out, a work for often changing the position of the camera is complicated.
By using a camera having a wide-angle lens (that is, a camera having a great angle of view), the visual field range is increased. Also in the case in which a hitting golf player take turns or a golf club to be used is changed, therefore, it is not necessary to often change the position of the camera.
However, a ball in an image obtained by the camera having a great angle of view is reduced. Accordingly, in the case in which an operator specifies a predetermined point of the ball image through an input pen or the like to measure a spin rate, there is a problem in that precision in the pointing is deteriorated. Moreover, the image obtained by the camera having a great angle of view has a considerable distortion in a peripheral portion (a portion having a great distance from the center of the image). Therefore, true coordinates are considerably shifted from apparent coordinates on the image so that the precision in a measured value becomes insufficient in some cases. In many cases, furthermore, the ball is not positioned on the center of the image in the photographing using the camera having a great angle of view. For this reason, the front part of the ball cannot be photographed but the ball is photographed in an oblique direction. Thus, the true coordinates are considerably shifted from apparent coordinates on the image so that precision in the measured value becomes insufficient.